To perform high density automatic surface mounting with respect to a circuit board using a chip mounter (automatic chip mounting machine), electronic component elements such as resistive elements, capacitors and inductors are made small in size, and the elements are coated with a resin (normally thermosetting resin) or the like as an outer packaging material to thereby form a cylindrical or rectangularly shaped chip.
In this respect, as a winding type chip inductor in which a coil is wound around the core portion of a core, a spiral inductor having a structure in which a coil is wound around the core portion of a double enveloping core (barbell-like core) and soldered to external electrodes made of metallic plates provided on flange portions on both ends of the coil, respectively, is well known. With an inductor of this type, however, possible damage to the inductor may occur during mounting or handling, due to the exposed coil.
In view of this deficiency, a resin molded chip inductor 10, as shown in FIGS. 5(A) and 5(B) is proposed. The resin molded chip inductor is manufactured by injection molding resin 9 as an outer packaging material to entirely surround a chip inductor element formed by soldering, and connecting the ends of a coil 8 wound around the core portion 1 of a double enveloping core 7 to external electrodes 5 comprised of metallic lead frames and provided on both flange portions 2, 2 of the double enveloping core 7, respectively, whereby part of the external electrodes 5 form a rectangular outer shape by curving the tip ends of the external electrodes 5 into an L-shape.
The double enveloping core 7 used for the above chip inductor 10 is made of a magnetic material such as high resistance nickel-zinc ferrite or an insulating material such as alumina. The resin 9 as the outer packaging material is epoxy synthetic resin formed by, for example, injection molding. The coil 8 is an insulating material coated conductor (wherein the insulating coating material is polyurethane or polyamideimide) having a diameter of about 0.05 to 0.2 mm. Depending on the purpose, one single wire or paired wires may be used for the coil 8.
As shown in FIG. 6, the core portion 1 of the double enveloping core 7 is formed between external electrodes 15 which have directly bonded electrode structure thereon by printing and baking conductor paste to flange portions 2, 2 on both ends of the core portion 1. A resin coating material (normally thermosetting resin coating material) 14 is coated around the coil 8 as an outer packaging and the resin coating material 14 thus coated on the coil 8 is heated and hardened. Through this process, a very small chip inductor 20 can be made.
In addition to the above proposed manufacturing method for the chip inductor 10 shown in FIG. 5 comprising injection-molding resin 9 serving as an outer packaging material, as shown in FIG. 6, there is proposed a manufacturing method comprising coating a resin coating material 14 serving as an outer packaging material stored in a coating material pan 23 on the periphery of a chip inductor element 11, having the core portion 1 of a double enveloping core 7 put crosswise and the end portions of the coil 8 soldered and connected to directly bonded external electrodes 15, provided on both flange portions of the core portion 1 by means of a coater 27, by rotation of the coating disk 24. Meanwhile, the element is held by a product chunk 22 and rotated by a rotating drum disk 21, and heating and hardening of the resin coating material 14 thus coated is carried out, finally plating the external electrodes.
Namely, as described in the flowchart (shown as FIGS. 7(A)-7(F)), the chip inductor 20 is manufactured by sequentially conducting the following steps:                (a) forming a double enveloping core 7 (FIG. 7(A));        (b) sintering the core 7 (FIG. 7(B));        (c) forming external electrodes (15) directly bonded to flange portions 2, 2 on both ends of the core 7 by printing and baking conductor paste such as silver, silver-platinum or copper (FIG. 7(C));        (d) winding the coil 8 around a core portion 1, and soldering both ends of the core 7 to the external electrodes (15), respectively, thereby forming an inductor element 11 (FIG. 7(D));        (e) coating the coil 8 with an epoxy synthetic resin coating material to form a heat resistant resin coating material 14 (FIG. 7(E)); and        (f) plating the external electrodes 15 to form a plated layer 17 by tin plating, nickel plating, solder plating or a combination thereof (this step may be omitted in certain cases) (FIG. 7(F)).        
A conventional chip inductor 10, wherein the entire element is coated with resin, becomes considerably larger in size than the outer dimensions of the element. Therefore, a conventional electronic chip component of this type is not suited to be made small in size.
With an electronic chip component such as the above-stated chip inductor 20, downsizing of the electronic component almost in compliance with a layered magnetic capacitor and the formation of the electronic component into a chip is possible, but in the outer packaging formation step, the coat formed by coating the resin coating material 14 around the coil 8 by means of the coater 27 becomes barrel-shaped having a swollen central portion as indicated by reference numeral 20, as shown in FIG. 6. This makes it difficult to stably mount these components during surface mounting and results in an increase in the outer dimension of the element. Thus, the barrel shape is not preferable for making an electronic chip component small in size.
In the case of a resin mold type chip conductor, by contrast, which uses a so-called injection molding manufacturing method for arranging an element in a mold formed into a chip shape in almost the same dimensions as a desired element and injecting resin into the mold at high pressure, the resin is directly sprayed onto the element main body at high pressure. During this process, the injected resin strongly strikes against a wound coil 8 portion and an irregular winding disadvantageously tends to occur. Further, when injection molding is conducted using thermosetting resin, it becomes difficult to recycle the resin on a runner portion, and material cannot be effectively used. Besides, when a small gap exists between the element within the mold and the inner wall of the mold, e.g., when the coil is fully wound around the core put between both flange portions up to the outer dimensional limit, there is a fear that resin may not be sufficiently filled deep within the mold.
Moreover, there are cases where magnetic powder containing resin (which normally contains magnetic powder of 55% or less by weight) is used as an outer packaging material so as to enhance the magnetic characteristics of the chip inductor. To form resin having a high magnetic powder content (75% by weight or more) into an outer package, there is no avoiding limiting the dimensions of the coil and ensuring an outer packaging material having a certain thickness around the coil, which are disadvantageous for making the inductor smaller in size and making direct current resistance low. It is particularly disadvantageous to the conventional chip conductor using a core having rectangular flanges and a rectangular core portion.